An electrostatographic printing machine such as a photocopier, laser printer, facsimile machine or the like employs an imaging member that is exposed to an image to be printed. Exposure of the imaging member records an electrostatic latent image on it corresponding to the informational areas contained within the image to be printed. The latent image is developed by bringing a developer material into contact therewith. The developed image is transferred to a support material such as paper either directly or via an intermediate transport member. The developed image on the support material is generally subjected to heat and/or pressure to permanently fuse it thereto.
Many types of developer compositions, including both dry developer compositions and liquid developer compositions, have been proposed for use in the development of latent electrostatic images. Dry developer compositions typically suffer from the disadvantage that distribution of the dry toner powder contained therein on the surface of the element bearing the latent image is difficult to control. These dry developers have the further disadvantage that the use thereof may create excessive amounts of dust and that high resolution is often difficult to obtain due to the generally relatively large size of the dry developer powder particles.
Many of the disadvantages accompanying the use of dry developer compositions have been avoided by the use of liquid developers. Liquid developers have a number of advantages over the use of dry developers. Because liquid developers contain smaller toner particles than dry developers, they produce higher resolution images. As liquid developers are pumped through tubing within the machines there are no dusting problems that commonly arise with the use of dry developers. Additionally, because liquid developers are not tribocharged, they are less sensitive to humidity. Liquid developers are usually comprised of an electrically insulating liquid which serves as a carrier and which contains a stable dispersion of charged particles known as toner particles comprising a pigment such as carbon black, generally associated with a resinous binder, such as, for example, an alkyd resin. A charge control agent is often included to stabilize the magnitude and polarity of the charge on the toner particles. In some cases, the binder itself serves as a charge control agent. Liquid developers can also have soluble ionic material in solution known as charge directors which impart a charge on the toner particles.
To achieve suitable physical stability of the toner particles dispersed in conventional liquid electro-graphic developers, any of several types of various "stabilization" additives are incorporated to prevent the toner particles from settling out of the carrier liquid. However, stabilized liquid developer compositions tend to become "deactivated" within a few weeks and the toner particles tend to agglomerate or settle out of the developer. Consequently, the resultant liquid developer composition containing conventional liquid developer toner particles tends to become incapable of producing electrostatic prints of good quality and density. Once the toner particles settle out of the developer suspension, it is often difficult to redisperse them, and, even if redispersed, it is often found that the redispersed liquid developer does not possess the same developer characteristics as the original developer.
Because stabilization in liquid developers has been and is still a difficult problem to overcome, liquid developers are often prepared in the form of so-called "concentrates", i.e., mixtures of resins, pigments and/or dyes with a low liquid content. (See Santilli, U.S. Pat. No. 4,052,325.) These concentrates are stable and exhibit a relatively long shelf life. The loss of stability which occurs in conventional liquid developers, as noted hereinabove, occurs primarily in the diluted form of the concentrate which is the "working" form of the developer, i.e., the form of developer composition actually used in most electrographic developing processes.
Stability in "working" liquid developer compositions may be improved to some extent by the use of the various stabilization agents disclosed in U.S. Pat. No. 2,899,335 (York). These additives are most effective in a developer when used in conjunction with toner particles having a very small particle size. However, even in these situations where stability is achieved in working developers, this stability is often accompanied by too high a developer sensitivity which tends to result in a high degree of background density in the resultant liquid developed electrographic images.
Another problem associated with conventional "stabilized" liquid developers has been the problem of replenishment. Once the developer is used to produce a number of developed images, the developer becomes depleted of toner particles and must be replenished.
In addition to the "stabilized" liquid developers described above, various "redispersible" liquid developers have been formulated which are characterized by toner particles which, upon settling out of suspension with the liquid carrier vehicle of the developer, are readily redispersed in the liquid carrier and, when so redispersed, exhibit developer characteristics similar to the original developer. However, various problems still exist with many of these "redispersible" developers. For example, the toner particles of many of these developers cannot be readily fixed, except to rough-surfaced toner image receiving sheets such as conventional zinc oxide coated papers, using preferred fixing temperatures of about 100.degree. C. or less. These developers, therefore, cannot be employed, except with further binder addenda, in various transfer processes because these processes use smooth surfaced toner-image receiving elements, such as dielectric resin-coated papers, i.e. papers coated with a film-forming dielectric resin. Still other available redispersible developers, although redispersible at ordinary room temperatures, exhibit pronounced caking or agglomeration of the toner particles when subjected to extended periods of storage (e.g., 24 hours) at temperatures above room temperature, and cannot be readily dispersed.
U.S. Pat. No. 4,052,325 (Santilli) discloses a liquid developer containing heat-fixable toner particles, wherein the toner particles contain a linear polyester polymer. The polyester polymer may have a structural formula as follows: ##STR1##
Diols and dicarboxylic acids may be used to prepare the polyester polymer. The diols may include aliphatic, alicyclic, and aromatic diols such as bisphenols, alkylene glycols or monocyclic and polycyclic diols. The dicarboxylic acids may include aliphatic, alicyclic and aromatic dicarboxylic acids, acid anhydrides and acid halide salts. A process of preparing the liquid developer comprises the steps of: (1) dissolving the polyester polymer in a suitable solvent in a ball mill wherein a pigment or other additives may be added forming a polymer-solvent mixture; (2) separating the mixture from the milling beads and the solvent; and (3) grinding the resulting dry polymer-containing material in a ball mill with a small amount of a liquid carrier vehicle creating a developer concentrate.
U.S. Pat. No. 4,659,640 (Santilli) discloses a liquid electrographic developer containing polyester based toner particles and special waxes. Preferred polyester binders have recurring diacid-derived units having the formula: EQU --O--G.sup.1 --O--
wherein G.sup.1 represents straight or branched-chain alkylene having about 2 to 12 carbon atoms or cycloalkylene, cycloalkylenebis(oxyalkylene) or cycloalkylene-dialkylene; and aliphatic, alicyclic or aromatic dicarboxylic acid recurring units which preferably contain sulfur. A process of preparing the liquid developer comprises the steps of: (1) melt-blending the polyester binder and a wax at a temperature above the melting temperature of the amorphous polyester; (2) cooling the blend; (3) pulverizing the blend; and (4) dispersing the blend in a volatile carrier.
U.S. Pat. No. 4,812,377 (Wilson et al.) discloses dry or liquid developers having finely divided toner particles comprising a fusible branched chain polyester resin which contains 2,3,-dihydro,1,3-dioxo-2-yl-1H-isoindole-ar(yl or -diyl) groups as chain capping or backbone groups of the polyester. The toner compositions can be ground to a very small particle size. The polyesters for the toner compositions are prepared by polymerization of polyester monomers such as dicarboxylic acids and diols. The diols may include neopentyl glycol. The polyester may include various polyols to create a polyester branching chain. Branching may be created by including polyols with the polyester monomers. These may include, e.g., trimellitic anhydride. A process of preparing the solid polyester polymer composition comprises the steps of: (1) crushing the polymer and then melt-blending with a colorant; (2) cooling and solidifying the blended composition; (3) crushing and coarsely grinding the composition in a mechanical mill; and (4) pulverizing the coarsely ground composition to a desired small toner particle size.
U.S. Pat. No. 5,006,441 (Kato) discloses a liquid developer comprising a resin dispersed in a non-aqueous solvent, wherein the resin is a copolymer resin obtained by polymerizing a solution containing at least one monofunctional monomer and at least one resin which is a polymer having at least a recurring unit having a formula: ##STR2## wherein X.sup.1 represents --COO--, --OCO--, --CH.sub.2 OCO--, CH.sub.2 COO--, --O--, --SO.sub.2 --, R.degree. represents a hydrocarbon group having from 6 to 32 carbon atoms and a.sup.1 and a.sup.2, which may be the same or different, each represents a hydrogen atom, a halogen atom, a cyano group, a hydrocarbon group having from 1 to 8 carbon atoms, or --COO--Z.sup.1 or --COO Z.sup.1 bonded via a hydrocarbon group having from 1 to 18 carbon atoms (wherein Z.sup.1 represents a hydrogen atom or a hydrocarbon group having from 1 to 18 carbon atoms). The monomers used in forming the resin may include a polyhydric alcohol such as neopentyl glycol.
Currently envisioned liquid developer printing machines require high solids replenishment to minimize the buildup of excess liquid carrier in the machine. This is because the liquid carrier and the toner are depleted at uneven rates depending on the amount of toner solids taken by each image, the degree to which carrier fluid imbibes into toner solids, the rate at which the paper or receiver sheet absorbs carrier fluid, and the rate at which carrier fluid is lost by evaporation. Theoretically, all carrier fluid is permanently contained in the printing machine and steps are taken to eliminate carrier losses.
Where image density is high, large quantities of toner solids are used while fluid loss is virtually zero. As toner solids are depleted, the volume of the bath changes negligibly. Replenishing the bath with toner concentrate at 10% solids, for example will cause the volume of the bath to grow very quickly, since 9 parts fluid are being added with every one part solids. Every added liter of concentrate causes the bath volume to grow nearly one liter. Consequently, the excess fluid must be removed, at considerable expense. As the efficiency of carrier fluid containment increases, it becomes necessary to replenish the developer with concentrates of increasingly higher concentration to prevent bath growth. However, desirably high concentrations have not previously been attained.
U.S. Pat. No. 3,397,254 (Wynstra et al.) (hereby incorporated by reference) discloses carboxyl terminated polyesters made by reacting a hydroxyl terminated polyester with a tricarboxylic acid anhydride. The hydroxyl-terminated polyester is a reaction product of a dicarboxylic cyclic acid or anhydride thereof and a dihydric compound. The dicarboxylic cyclic acid has the formula: EQU R(COOH).sub.2
wherein R is a cyclic hydrocarbon radical having at least 4 carbon atoms and includes terephthalic acid, isophthalic acid, and cyclohexane dicarboxylic acid and anhydrides thereof. The dihydric compound is a glycol, phenol, cycloaliphatic diol, or ether diol and includes ethylene glycol, butane diol, and neopentyl glycol. The hydroxyl-terminated polyester is prepared by admixing the monomers such that the dihydric compound is present in stoichiometric excess. The tricarboxylic acid anhydride includes trimellitic acid anhydride. The product has a degree of polymerization of at least 3 and a carboxyl functionality of at least three.
U.S. Pat. No. 4,275,189 (Danick) (hereby incorporated by reference) discloses thermosetting powder coating resins having good wear resistance comprising an oligomer of neopentyl glycol or cyclohexane dimethanol, and terephthalic acid, isophthalic acid or dimethyl terephthalate. The oligomer is reacted with trimellitic anhydride to provide a trimellitate. The trimellitate is reacted with a dicarboxylic anhydride or acid to form a thermosetting crosslinkable resin with defined viscosity and acid values.
U.S. Pat. No. 5,006,612 (Danick et al.) (hereby incorporated by reference) discloses powder coating compositions made of linear polyesters which have a carboxylated polyester resin comprising a reaction product of an aliphatic dicarboxylic acid which has 2 to 9 carbon atoms, and a first hydroxyl terminated polyester. The first polyester is a reaction product of not more than about 53 weight percent, based upon a reaction mixture for the first polyester, of terephthalic or isophthalic acid or mixtures thereof, and neopentyl glycol and/or cyclohexane dimethanol. After the carboxylated polyester is formed, it is allowed to cool and solidify. The solidified resin is crushed or granulated and blended in an extruder with a polyepoxide, pigments and other additives to provide a mixture. The mixture is then cooled, crushed, finely ground and sieved. The carboxylated polyester resins are crosslinked in use by heating or baking with an epoxy resin.
U.S. Pat. No. 4,740,580 (Merck et al.) discloses a process of preparing carboxyl group-terminated polyesters for a powdered thermosetting coating composition suitable for application as paint or varnish on electricity conducting articles by electrostatic powder spray methods and fluidized bed coating processes. The carboxyl group terminated polyester is homogeneously mixed with an epoxy compound containing at least two epoxy groups. The process comprises the step of reacting in one step at elevated temperatures terephthalic acid and at least one dihydric aliphatic compound and optionally an aromatic polycarboxylic acid having three or more carboxyl groups and/or a polyhydric organic compound having three or more hydroxyl groups and/or linear aliphatic or cycloaliphatic dicarboxylic acid. The dihydric aliphatic compound may be neopentyl glycol. The resulting carboxyl group-terminated polyester is then cast into a thick layer and allowed to cool. The cooled carboxyl group terminated polyester is ground to give particles. The carboxyl group-terminated polyester particles are cross-linked with epoxy resins which are homogeneously mixed in a kneader or a twin screw extruder. The extruded mixture is ground and sieved. Other substances may be added to the mixture such as pigments and flow control agents